Thermoplastic olefins (TPOs), impact copolymers (ICPs) and thermoplastic vulcanizates (TPVs), collectively referred to herein as “heterogeneous polymer blends”, typically comprise a crystalline thermoplastic phase and a high molecular weight or crosslinked elastomeric phase. These heterogeneous polymer blends also commonly include non-polymeric components, such as fillers and other compounding ingredients. The heterogeneous polymer blends have multiphase morphology where a thermoplastic, such as isotactic polypropylene (often referred as the hard phase), forms a continuous matrix phase and the elastomeric component (often referred as the soft phase), generally derived from an ethylene containing copolymer, is the dispersed component. The polypropylene matrix imparts tensile strength and chemical resistance to the blend, while the ethylene copolymer imparts flexibility and impact resistance.
TPOs and ICPs are typically made during the polymerization process by differential polymerization of the polymer components, although some can also be made by mechanical blending. TPVs are also blends of thermoplastic and elastomer, like TPOs, except that the dispersed elastomeric component is crosslinked or vulcanized in a reactive extruder during compounding. Crosslinking of the elastomeric phase generally allows dispersion of higher amounts of rubber in the polymer matrix, stabilizes the obtained morphology by preventing coalescence of rubber particles, and enhances mechanical properties of the blend.
Traditionally, the elastomeric component in heterogeneous polymer blends has been provided by highly amorphous, very low density ethylene-propylene copolymers (EP) and ethylene-propylene-diene terpolymers (EPDM) having a high molecular weight expressed in Mooney units. Recently, other ethylene-alpha olefin copolymers have been used, especially very low density ethylene-butene, ethylene-hexene and ethylene-octene copolymers which generally have a lower molecular weight expressed in Melt Index units. The density of these latter polymers is generally less than 0.900 g/cm3, indicative of some residual crystallinity in the polymer.
The major market for TPOs is in the manufacture of automotive parts, especially bumper fascia. Other applications include automotive interior components such as door skins, air bag covers, side pillars and the like. These parts are generally made using an injection molding processes. To increase efficiency and reduce costs, it is necessary to decrease molding times and reduce wall thickness in the molds. To accomplish these goals, manufacturers have turned to high melt flow polypropylenes (Melt Flow Rate>35 dg/min.). These high melt flow rate (MFR) resins are low in molecular weight and consequently difficult to toughen, resulting in products that have low impact strength. It would be desirable to have a polymer blend with greater elongation to break and more toughness, improved processability, and/or a combination thereof.
In addition, in-reactor blends have been sought as an alternative to physical blending since in-reactor blends offer the possibility of improved mechanical properties through more intimate mixing between the hard and soft phases, through the generation of hard/soft cross products, as well as lower production costs. Use of compatibilizer is another way to improve interfacial tension between hard and soft phases in the heterogeneous blend, thereby improving the mechanical properties.
For example, Datta, et al [D. J. Lohse, S. Datta, and E. N. Kresge, Macromolecules 24, 561 (1991)] describe EP backbones functionalized with cyclic diolefins by terpolymerization of ethylene, propylene and diolefin. The statistically functionalized EP “soft block” is then copolymerized with propylene in the presence of a Ziegler-Natta catalyst capable of producing isotactic polypropylene. In this way, some of the “hard” block polypropylene chains are grafted through the residual olefinic unsaturation onto the EP “soft” block previously formed. See also, EP-A-0 366411. U.S. Pat. No. 4,999,403 describes similar graft copolymer compounds where functional groups in the EPR backbone are used for grafting isotactic polypropylene having reactive groups. In both the graft copolymers are said to be useful as compatibilizer compounds for blends of isotactic polypropylene and ethylene-propylene rubber. A limitation of this class of reactions, in which chains with multiple functionalities are used in subsequent reactions, is the formation of undesirable high molecular weight material typically referred to as gel in the art.
U.S. Pat. No. 6,147,180 discloses a thermoplastic elastomer composition comprising a branched olefin copolymer backbone and crystallizable side chains, wherein the copolymer has A) a Tg as measured by DSC less than or equal to 10° C.; B) a Tm greater than 80° C.; C) an elongation at break of greater than or equal to 300%; D) a tensile strength of greater than or equal to 1,500 psi (10.3 MPa) at 25° C. and E) an elastic recovery of greater than or equal to 50%. The thermoplastic elastomer composition can be produced by A) polymerizing ethylene or propylene, optionally with one or more copolymerizable monomers, in a polymerization reaction under conditions sufficient to form a polymer having greater than 40% chain end-group unsaturation; and B) copolymerizing the product of A) with ethylene and one or more copolymerizable monomers so as to prepare said branched olefin copolymer. The two polymerization steps can be conducted sequentially or concurrently.
U.S. Pat. No. 6,114,457 describes a high melt strength polyethylene composition having a polydispersity index of less than or equal to 3, an average branching index (g′) as measured by GPC/V is of at least 0.9, wherein the composition comprises A) branched polyethylene copolymers prepared by insertion polymerization of ethylene, ethylene-containing macromers, and optionally, additional copolymerizable monomers, and B) essentially linear ethylene copolymers.
U.S. Pat. No. 6,423,793 describes a thermoplastic elastomer composition comprising: (a) isotactic or syndiotactic polymer branches derived from macromers that are prepared by a process comprising contacting, in solution, at a temperature from about 90° C. to about 120° C., propylene monomers with a catalyst composition comprising a chiral, stereorigid transition metal catalyst compound capable of producing isotactic or syndiotactic polypropylene; and (b) an atactic polymer backbone prepared by a process comprising copolymerizing the macromers with propylene and, optionally, one or more copolymerizable monomers, such as ethylene, in a polymerization reactor using an achiral transition metal catalyst capable of producing atactic polypropylene.
U.S. Pat. No. 6,660,809 describes a polyolefin product which comprises a branched olefin copolymer having an isotactic polypropylene backbone, polyethylene branches and, optionally, one or more comonomers. The total comonomer content of the branched olefin copolymer is from 0 to 20 mole percent. Also, the mass ratio of the isotactic polypropylene to the polyethylene ranges from 99.9:0.1 to 50:50. The copolymer is produced by a process which comprises: a) copolymerizing ethylene, optionally with one or more copolymerizable monomers, in a polymerization reaction under conditions sufficient to form copolymer having greater than 40% chain end-group unsaturation; b) copolymerizing the product of a) with propylene and, optionally, one or more copolymerizable monomers, in a polymerization reactor under suitable polypropylene polymerization conditions using a chiral, stereorigid transition metal catalyst capable of producing isotactic polypropylene; and c) recovering a branched olefin copolymer.
More recently, a process has been proposed for producing olefin block copolymers by a so called chain shuttling polymerization, in which growing polymer chains are thought to be passed between catalyst sites with the assistance of a metal alkyl complex, so that portions (or blocks) of a single polymer molecule are synthesized by at least two different catalysts. See, Arriola et al. “Catalytic Production of Olefin Block Copolymers via Chain Shuttling Polymerization”, Science, May 5, 2006, Vol. 312. The resultant block copolymers are thought to be linear and are reported to have good elastomeric properties at temperatures higher than those of traditional random copolymers of similar density, suggesting that these block copolymers should be useful in high-temperature applications. However, although the process has been demonstrated for copolymers comprising propylene/ethylene “hard” blocks and ethylene/octene “soft” blocks, it is not currently clear that the process is applicable to copolymers comprising isotactic polypropylene “hard” blocks.
U.S. Pat. No. 7,223,822 describes a process for producing branched polymers including at least 50 mol % C3 to C40 olefins, wherein the process includes: (1) feeding a first catalyst, an activator, and one or more C2 to C40 olefins into a first reaction zone at a temperature of greater than 70° C. and a residence time of 120 minutes or less to produce a product; (2) feeding the product, a second catalyst, and an activator into a second reaction zone at a temperature of greater than 70° C., and a residence time of 120 minutes or less. One of the catalysts should be chosen to produce a polymer having a weight average molecular weight of 100,000 or less and a crystallinity of 20% or less. The other catalyst should be chosen to produce a polymer having a weight average molecular weight of 100,000 or less and a crystallinity of 20% or more.
US Patent Application Publication No. 2006/0293455 discloses an in-reactor heterogeneous polymer blend comprising (a) a continuous phase comprising a thermoplastic first polymer having a crystallinity of at least 30%; and (b) a dispersed phase comprising particles of a second polymer different from the first polymer dispersed in said continuous phase and having an average particle size less than 1 micron. The second polymer has a crystallinity of less than 20% and is at least partially crosslinked, making the blend useful as a thermoplastic vulcanizate. The blend is said to contain branched block copolymers comprising an amorphous backbone having crystalline side chains originating from the first polymer.
In our co-pending U.S. patent application Ser. No. 12/335,252, filed Dec. 15, 2008) we have disclosed an in-reactor polymer blend comprising (a) a propylene-containing first polymer; and (b) propylene-containing second polymer having a different crystallinity from the first polymer, wherein the polymer blend has a melting temperature, Tm, of at least 135° C., a melt flow rate of at least 70 dg/min, a tensile strength of at least 8 MPa, an elongation at break of at least 300%. Thus this blend exhibits a unique combination of a high melt flow rate combined with high tensile strength, tear strength and elongation at break, making it attractive for injection molding applications and particularly for injection molding components having a scratch resistant skin.
Certain applications of TPOs, such as in automotive fascia panels, require a unique, and difficult to achieve, set of properties including injection moldability, high strength, good scratch resistance and good grain retention. As a result, most TPOs used for these applications are styrenic block copolymers (SBC), typically linear triblock polymers, such as styrene-isoprene-styrene and styrene-butadiene-styrene, such as Kraton G 1650. These copolymers typically are prepared by sequential anionic polymerization or by chemical coupling of linear diblock copolymers. However, SBC copolymers are relatively expensive compared to TPOs based on ethylene and propylene. Also the glass transition temperature (Tg) of the styrenic block copolymer is typically less than or equal to about 80-90° C., thus presenting a limitation on the utility of SBC copolymers under higher temperature conditions.
The present invention seeks to provide an in-reactor polymer blend which is based on ethylene and propylene and which exhibits improved injection moldability, strength, scratch resistance and grain retention properties making the blend an attractive alternative to SBCs in automotive fascia applications. This in-reactor polymer blend has crystallinity from both long sequences of methylenes (polyethylene type crystallinity) and long sequences of isotactic propylenes (polypropylene type crystallinity). Consequently, it has a much higher upper use temperature than SBC copolymers. Surprisingly, it has very similar strain at break and stress at break when compared to SBC copolymers.